1. Field of the Invention
The invention relates to an echographic probe with sector scanning using a transducer capable of coming into contact with the structure to be examined.
It is notably but not exclusively applied to echography of eye structures.
2. Description of the Prior Art
Generally, it is known that in opthalmology, 2-D echography at 10 MHz is used in current practice for exploring the anatomy and pathologies of eye structures and more particularly that of the posterior pole (retina, optical nerve, vitreous).
The technique of the probes uses sector scanning, with which probes may be obtained of small sizes, comparatively to bar probes.
In order to perform this type of scan, echographic probes are used, including an open tubular casing in its front portion and the inner volume of which is divided into two compartments by a sealed partition, i.e.:                a rear compartment which extends on the side of the bottom of the casing: this compartment comprises motorization and power supply circuits for the control and processing parts of the apparatus,        a front compartment adjacent to the aperture of the casing, which contains a mobile transducer as well as all or part of its actuation mechanism.        
The aperture of the front compartment is usually closed by a flexible or hard sealed membrane, in order to obtain a sealed interior space containing a coupling liquid which should have high transmissibility for ultrasonic waves.
This membrane which is intended to come into contact with the eye of the patient, should be made in a biocompatible material which does not damp high frequency ultrasound. It confines the coupling liquid while allowing the eye to be protected from any accidental contact with mechanical parts, i.e., notably the transducer and/or its actuation mechanism. Now, these are results which are difficult to obtain, which explains the invention's advantage of reducing these problems to the making of the transducer itself, i.e., at the source of ultrasound, so as to be able to act more easily on the global features of the transducer.
Echographic apparatuses of this type nevertheless prove to have a certain number of drawbacks. Indeed:
Before reaching the eye, the ultrasonic wave focused by the concave curvature of the piezoelectric element of the transducer, has to pass through several layers of material having different properties and this, with different propagation velocities, the path of these waves inside these layers varying according to the (variable) position of the transducer.
This causes variations in the focal length of the transducer and inaccuracies in the obtained echographic image.
Moreover, it is seen that in the case of a high frequency transducer (from 15 MHz), liquids and membranes become increasingly absorptive. Consequently, the frequencies which effectively penetrate the tissues are much less than the frequencies emitted by the transducer.
Thus, as an example, for a transducer emitting with a frequency of 20 MHz, the transmitted central frequency will be 18 MHz with a loss of 10 dB.
Within the scope of open probes, all these drawbacks are suppressed since the coupling medium consists in a water bath which has good acoustic properties. However, the transducer performs its motion at a very small distance from the structure to be examined and even if it has a circular shape, its edges are aggressive and accordingly, it is not possible to suppress any risk of trauma by accidental contact of the transducer with the eye (scratching of the cornea by the circular edge of the transducer).